Modelling the influence of single acid and mixture on bacterial growth

Depending on environmental factors, the prediction of bacterial growth is made difficult by the complexity of foodstuff. Although the influence of temperature, pH, and water activity are usually taken into account, models have to be completed with the influence of acid mixture. Nine strains of Listeria spp., four Salmonella spp., one Staphylococcus aureus, one Escherichia coli, and Listeria innocua ATCC 33090 were used for this study to extend model proposed by [Le Marc, Y., Huchet, V., Bourgeois, C., Guyonnet, J., Mafart, P., Thuault, D., 2002. Modelling the growth kinetics of Listeria as a function of temperature, pH and organic acid concentration. International Journal of Food Microbiology 73, 219–237]. Derived from data of [Houtsma, P.C., Kusters, B.J., De Wit, J.C., Rombouts, F.M., Zwietering, M.H., 1994. Modelling growth rates of Listeria innocua as a function of lactate concentration. International Journal of Food Microbiology 24, 113–123] and our own data, the extended model described accurately different effects of addition of acid salts in the medium (decrease of water activity and pH, variation of undissociated weak acid form, and variation of synergetic effect between environmental factors). This previous model was implemented to describe the observed variability of behaviour of the different studied strains. reflected the general behaviour of species (sensitiveness to low or high undissociated acid concentration), and MIC_U reflected the various resistances of strains. From this simple model, a new model was built for describing the effects of concentrations of several mixed acids on bacterial growth rates. Simulations of growth were carried out from three acids mixtures by inputting parameter estimates previously obtained. Despite a very variable effect of investigated acids on growth, the new model yielded fair predictions.     

Strain variability of the behavior of foodborne bacterial pathogens: A review

Differences in phenotypic responses among strains of the same microbial species constitute an important source of variability in microbiological studies, and as such they need to be assessed, characterized and taken into account. This review provides a compilation of available research data on the strain variability of four basic behavioral aspects of foodborne bacterial pathogens including: (i) virulence; (ii) growth; (iii) inactivation; and (iv) biofilm formation. A particular emphasis is placed on the foodborne pathogens Listeria monocytogenes and Salmonella enterica. The implications of strain variability for food safety challenge studies and microbial risk assessment are discussed also. The information provided indicates that the variability among strains of foodborne bacterial pathogens with respect to their behavior can be significant and should not be overlooked. However, in order for the mechanisms underlying the observed strain variability to be elucidated and understood, phenotypic variability data, such as those reviewed here, should be evaluated in conjunction with corresponding findings of studies assessing the molecular/physiological basis of this variability.     

Staphylococcal growth and enterotoxin production in the presence of meat cultures (non LAB)

The effect of meat cultures (non lactic acid bacteria) on the growth and production of enterotoxins and thermonuclease by Staphylococcus aureus was studied. Micrococcus varians did not affect growth nor the synthesis of metabolites. Levels of enterotoxins A, B and D produced by the respective S. aureus strains were reduced by S. xylosus, S. saprophyticus and S. carnosus. The two latter species prevented production of enterotoxin C₁ and S. xylosus markedly reduced the amount produced. The three coagulase-negative staphylococci showed little inhibitory effect on the growth of S. aureus. Penicillium nalgiovense did not show inhibitory activity against the four S. aureus strains. Debaryomyces hansenii slightly inhibited growth of the enterotoxin A-producing strain, but reduced enterotoxin synthesis at 30 °C. Thermonuclease was detected whenever enterotoxins were detected though the influence of the effector organism was dependent on the test strain.     

Use of Bayesian modelling in risk assessment: application to growth of Listeria monocytogenes and food flora in cold-smoked salmon

An attempt to use a Bayesian approach to model variability and uncertainty separately in microbial growth in a risk assessment is presented. It was conducted within the framework of a French project aiming at assessing the exposure to Listeria monocytogenes in cold-smoked salmon. The chosen model describes the effect of time and temperature on bacterial growth. A Bayesian approach close to the one proposed by Pouillot et al. [Int. J. Food Microbiol. 81 (2003) 87] is used to estimate the variability and uncertainty of growth parameters from both literature data and data experimentally acquired during the project. Variability between strains and between products is taken into account. The growth of the food flora of cold-smoked salmon is also modelled by the same method. The results obtained for both models are used to predict the simultaneous growth of L. monocytogenes and food flora in cold-smoked salmon with a competitive model, expressing variability and uncertainty through a second-order Monte Carlo simulation.     

Effect of the growth environment on the strain variability of Salmonella enterica kinetic behavior

Intra-species variability of microbial growth kinetic behavior is an event with important implications for food safety research. Aiming at the evaluation of the growth variability among Salmonella enterica strains as affected by the growth environment, the kinetic behavior of 60 isolates of the pathogen was assessed at 37 °C in tryptone soy broth of different pH values (4.3-7.0) and NaCl concentrations (0.5-6.0%). Maximum specific growth rate (μ_max) values corresponding to each strain and growth condition were estimated by means of absorbance detection times of serially decimally diluted cultures using the automated turbidimetric system Bioscreen C. A total of 9600 optical density curves were generated for the strains and the growth conditions tested. The variability of μ_max among the S. enterica strains was important and greater than that observed within the strains (i.e. among replicates). Moreover, strain variability increased as the growth conditions became more stressful both in terms of pH and NaCl. The coefficient of variation of μ_max among the tested strains at pH 7.0-0.5% NaCl was 6.1%, while at pH 4.3-0.5% NaCl and pH 7.0-6.0% NaCl was 11.8% and 23.5%, respectively. Beyond the scientific interest in understanding strain variability, the findings of this study should be useful in strain selection for exploitation in food safety challenge studies as well as in incorporating strain variability in predictive microbiology and microbial risk assessment.     

Temperature and water activity effects on growth and temporal deoxynivalenol production by two Argentinean strains of Fusarium graminearum on irradiated wheat grain

The ability of six strains of Pichia anomala, four strains of Pichia kluyveri and two strains of Hanseniaspora uvarum predominant during coffee processing to produce polygalacturonase (PG), pectin esterase (PE) and pectin lyase (PL) in yeast polygalacturonic acid medium (YPA) and in coffee broth (CB) was studied. For comparison, a reference strain of Kluyveromyces marxianus CCT 3172 isolated from cocoa and reported to produce high amount of PG was included.

Initial screening of PG activity using YPA medium showed that K. marxianus CCT 3172, P. anomala S16 and P. kluyveri S13Y4 had the strongest activity. Enzymatic assays showed that the four yeast species secreted PG, but none of the yeasts investigated was found to produce PE or PL. P. anomala S16 and P. kluyveri S13Y4 were found to produce higher amounts of PG when grown in CB than in YPA. When K. marxianus CCT 3172, P. anomala S16 and P. kluyveri S13Y4 were grown in YPA broth adjusted to pH of 3.0–8.0 and incubated at temperatures of 15–40 °C, the three yeast species secreted the highest amount of PG at pH 6.0 and at 30 °C. For PG secreted by K. marxianus CCT 3172 and P. anomala S16, the optimum pH and temperature for the enzymatic activity were 5.5 and 40 °C, respectively. On the other hand, PG produced by P. kluyveri S13Y4 showed the highest activity at pH 5.0 and 50 °C.

Significant differences in the extracellular activity of PG were found between the yeasts species as well as between strains within same species. High amounts of PG were produced by two strains of P. anomala and P. kluyveri. It is therefore likely that strains of those two species may be involved in the degradation of pectin during coffee fermentation.     

Effect of relative inoculum concentration on Listeria monocytogenes growth in co-culture

Growth suppression of multi-species bacterial populations in batch cultures by a single 'dominant' strain has been referred to in the literature as the 'Jameson Effect'. The effect is often attributed to production of specific inhibitors of growth by one species against another. To explore its basis, we hypothesised that the Jameson Effect is often a non-specific interaction and that growth inhibition of species in co-culture can be controlled by manipulation of inoculum concentration and growth rate so as to enable a selected species to achieve stationary phase first. The hypothesis was assessed by co-culturing pairs of bacterial species under conditions selected to ensure that both strains grew at a similar rate and manipulating the initial concentration of each species. Specifically, the effect of inoculum concentration on the growth kinetics of Listeria monocytogenes when co-cultured in complex laboratory media with Escherichia coli, Pseudomonas fluorescens or a commercial strain of the lactic acid bacterium, Lactobacillus plantarum, was studied. Starting numbers of the second strain were either higher than ( approximately 10(6) cfu mL(-1)), equal to ( approximately 10(4) cfu mL(-1)) or lower than ( approximately 10(3) cfu mL(-1)) L. monocytogenes. In most trials, the initial inoculum concentration governed which species became dominant and suppressed the growth of the other strain. L. monocytogenes was suppressed by all other strains when its inoculum level was lower. Conversely, when L. monocytogenes was initially present at higher concentration than either P. fluorescens or L. plantarum, their growth was suppressed. E. coli, however, was not suppressed by L. monocytogenes even when the E. coli density was initially lower. While simple competition for nutrients could explain most of the observations, in some co-culture experiments pH reduction also seemed to play a role in inhibition of growth of some species. In other cases, available growth substrates were apparently not utilised by both co-cultured species, and the Jameson Effect did not occur. Thus, while it appears that under many conditions the Jameson Effect may be largely due to non-specific inhibition, more complex interactions between co-cultured strains involving species-specific pH limits for growth and differential utilisation of growth substrates may also occur and confound the simple Jameson Effect.     

生物有机肥对烟草青枯病的防效及对土壤细菌群落的影响

烟草青枯病是一种危害严重的土传性病害,为了防治烟草青枯病,筛选了3株拮抗青枯雷尔氏菌的芽孢杆菌（甲基营养型芽孢杆菌JK3、枯草芽孢杆菌JK4和解淀粉芽孢杆菌JK10）,将其发酵液添加到有机肥中,经二次发酵后获得生物有机肥（SF2、SF4）,施入烟田。结果表明,生物有机肥能较好地防治烟草青枯病,同时促进烟株的生长,SF2、SF4处理的防效分别为82.18%、68.82%。同时,采用高通量测序技术分析了土壤细菌的群落结构,发现生物有机肥显著影响了土壤细菌的群落结构,促进了土壤中有益菌（如鞘氨醇单胞菌属、类芽孢杆菌属、耐热芽孢杆菌、梭菌属等）的增殖。芽孢杆菌和链霉菌的数量也明显增加,推测链霉菌和芽孢杆菌可抑制青枯雷尔氏菌的繁殖,进而控制烟草青枯病的发生。施用添加拮抗芽孢杆菌的生物有机肥是防治烟草青枯病的一项有效措施。     

Development of four strains of Sitophilus oryzae (L.) (Coleoptera: Curculionidae) on barley, corn (maize), rice, and wheat

Differences in development of four strains of Sitophilus oryzae (L.) (LS-2, Minnesota, Savannah, and Tanzania-90) on diets of hulled barley, yellow corn (maize), brown rice, and soft red winter wheat at 28°C and 55–60 r.h., were analyzed in terms of total progeny production, average time (days) to adult emergence, and rate of emergence with an analysis of variance. Significant main effects of strain and diet were obtained. Across all strains, progeny production was highest on barley and lowest on corn. All measured growth parameters indicated that corn was the least suitable diet of the tested cereals for this species. The Savannah strain of S. oryzae produced significantly more progeny and had shorter development times than the other strains. The strain from Tanzania produced smaller numbers of progeny, had significantly longer development times, and a significantly slower rate of emergence on wheat compared to that of the other strains. The Tanzania strain does not have the major amylase isozyme present in this species and has less than 50% of the total amylase activity found in the other three strains. It is postulated that the strong Tanzania × wheat interaction is caused by combination of naturally-occurring amylase inhibitors in ingested wheat with digestive amylase in this weevil strain and a subsequent decreased efficiency in food utilization.     

Comparison of primary predictive models to study the growth of Listeria monocytogenes at low temperatures in liquid cultures and selection of fastest growing ribotypes in meat and turkey product slurries

This study compared the performance of four primary mathematical models to study the growth kinetics of Listeria monocytogenes ribotypes grown at low temperature so as to identify the best predictive model. The parameters of the best-fitting model were used to select the fastest growing strains with the shortest lag time and greatest growth rate. Nineteen food, human and animal L. monocytogenes isolates with distinct ribotype were grown at 4, 8, and 12 degrees C in tryptic soy broth and slurries prepared from cooked uncured sliced turkey breasts (with or without potassium lactate and sodium diacetate, PL/SD) and cooked cured frankfurters (with or without PL/SD). Separate regressions were performed on semi-logarithm growth curves to fit linear (based on Monod) and non-linear (Gompertz, Baranyi-Roberts, and Logistic) equations and performance of each model was evaluated using an F-test. No significant differences were found in the performance of linear and non-linear models, but the Baranyi model had the best fit for most growth curves. The maximum growth rate (MGR) of Listeria strains increased with the temperature. Similarly MGR was found significantly greater when no antimicrobials were present in the formulation of turkey or frankfurter products. The variability in lag times and MGRs in all media as determined by the Baranyi model was not consistent among strains. No single strain consistently had the fastest growth (shortest lag time, fastest MGR, or shortest time to increase 100-fold), but nine strains were identified as fastest growing strains under most growth conditions. The lack of association between serotype and fastest strain was also observed in the slurry media study. The fastest growing strains resulting from this study can be recommended for future use in L. monocytogenes challenge studies in delicatessen meat and poultry food matrices, so as to develop conservative pathogen growth predictions.     

Multiple interactions between Streptococcus thermophilus, Lactobacillus helveticus and Lactobacillus delbrueckii strongly affect their growth kinetics during the making of hard cooked cheeses

In hard cooked cheeses, any interactions between the thermophilic starters as they grow during the cheese-making are critical, since they modify bacterial growth kinetics and acidification kinetics, so affecting the ripening process and the final characteristics of the cheese. Twenty-four experimental hard cooked cheeses were made under controlled conditions, the milk being inoculated with various combinations of thermophilic strains of Streptococcus thermophilus, Lactobacillus helveticus and Lactobacillus delbrueckii. Over the first day of manufacturing we recorded a wide range of different growth kinetics for each starter species used, and a wide range of pH kinetics, depending on the starter combination. Most of the bacterial variability could be statistically explained by the nature, quantity, and/or presence or absence of the different strains inoculated. Four main interactions between the three species were evidenced during cheese-making. There was antagonism between L. helveticus and L. delbrueckii. The lactobacilli had a positive effect on S. thermophilus, which was reciprocal for L. helveticus. L. helveticus had a negative effect on S. thermophilus cultivability. And the combination of S. thermophilus inoculated in large quantities and L. helveticus strain H2 had a negative effect on the growth of the L. delbrueckii strain D2. While the positive effect of L. delbrueckii on S. thermophilus probably corresponds to interactions in milk that have already been described and published, the other interactions were hitherto unknown. These interactions are of major importance for the growth kinetics of streptococci and thermophilic lactobacilli during cheese-making.     

Influence of storage conditions of apples on growth and patulin production by Penicillium expansum

Penicillium expansum causes blue mould rot, a serious post-harvest disease of apples, and is the main producer of the mycotoxin patulin. The present study aimed to determine the influence of storage conditions (i.e. temperature and O(2) level) on growth and patulin production by different P. expansum strains on a simulation medium and on apples. Growth was strongly influenced by the temperature, while the used atmosphere (20, 3, and 1% O(2); <1% CO(2)) had no effect. Optimal growth was observed at 25 degrees C for every strain tested. Patulin production was stimulated when the temperature decreased (from 20 to 10 or 4 degrees C), while a further decrease of the temperature to 1 degrees C caused a reduction in patulin production. The temperature at which the stimulation was changed into suppression was strain dependent. Similar results were observed for the O(2) level. A reduction of the O(2) level from 20 to 3% O(2) could stimulate or suppress patulin production depending on the strain, while a clear decrease of the patulin production was observed when the O(2) level was reduced from 3 to 1%. These results show that the induction of limited stress to the fungus, such as lowering the temperature or lowering the O(2) levels stimulates patulin production. However, the combination of different stress conditions (e.g. low temperature and low O(2)) will result in a reduced formation of the toxin. The combination of stress conditions, at which the transition from stimulation to suppression is observed, is strain dependent. Moreover, patulin production is characterized by a high natural variability. The presented results show that the temperature and O(2) level has to be as low as possible during the storage of apples in order to suppress patulin production and to guarantee food safety.     

Development and validation of a mathematical model to describe the growth of Pseudomonas spp. in raw poultry stored under aerobic conditions

Poultry meat spoils quickly unless it is processed, stored, and distributed under refrigerated conditions. Research has shown that the microbial spoilage rate is predominantly controlled by temperature and the spoilage flora of refrigerated, aerobically-stored poultry meat is generally dominated by Pseudomonas spp.

The objective of our study was to develop and validate a mathematical model that predicts the growth of Pseudomonas in raw poultry stored under aerobic conditions over a variety of temperatures.

Thirty-seven Pseudomonas growth rates were extracted from 6 previously published studies. Objectives, methods and data presentation formats varied widely among the studies, but all the studies used either naturally contaminated meat or poultry or Pseudomonas isolated from meat or poultry grown in laboratory media. These extracted growth rates were used to develop a model relating growth rate of Pseudomonas to storage or incubation temperature. A square-root equation [Ratkowsky, D.A., Olley, J., McMeekin, T.A., and Ball, A., 1982. Relationship between temperature and growth rate of bacterial cultures. J. Appl. Bacteriol. 149, 1–5.] was used to model the data. Model predictions were then compared to 20 Pseudomonas and 20 total aerobes growth rate measurements collected in our laboratory. The growth rates were derived from more than 600 bacterial concentration measurements on raw poultry at 10 temperatures ranging from 0 to 25 °C. Visual inspection of the data and the indices of bias and accuracy factors proposed by Baranyi et al. [Baranyi, J., Pin, C., and Ross, T., 1999. Validating and comparing predictive models. Int. J. Food Micro. 48, 159–166.] were used to analyze the performance of the model.

The experimental data for Pseudomonas showed a 4.8% discrepancy with the predictions and a bias of + 3.6%. Percent discrepancies show close agreement between model predictions and observations, and the positive bias factor demonstrates that the proposed model over-predicts growth rate, thus, can be considered fail-safe. Both Pseudomonas spp. as well as total aerobes may be considered good indicators of poultry spoilage.

A properly constructed and validated model for Pseudomonas growth under aerobic conditions can provide a fast and cost-effective alternative to traditional microbiological techniques to estimate the effects of storage temperature on product shelf-life. The model developed here may be used to determine the effect of both initial Pseudomonas concentration and storage temperature on shelf-life of poultry meat under aerobic storage conditions over temperatures from 0 to 25 °C.     

Effect of pH on ochratoxin A production by Aspergillus niger aggregate species

The effect of pH (2-10) on growth and ochratoxin A (OTA) production by 12 Aspergillus niger aggregate strains was studied in two culture media: Czapek yeast autolysate agar (CYA) and yeast extract sucrose agar (YES), over 30 days. The strains were selected to include different sources, different reported abilities to produce OTA and different ITS-5.8S rDNA RFLP patterns. YES was a better culture medium than CYA for OTA production. In this medium, OTA was produced from pH 2 or 3 to 10 depending on the strain. The results show the ability of A. niger aggregate strains not only to grow, but also to produce OTA over a wide pH range. The results will lead to a better understanding of the role of A. niger aggregate strains in the OTA contamination of several food commodities.     

Effect of growth temperature on yeast lipid composition and alcoholic fermentation at low temperature

The loss of viability of wine yeast strains due to low-temperature fermentations could be overcome by increasing their stress tolerance and adaptability. Changes in membrane lipid composition are one of the first responses to cold stress. The aim of this study was to analyze the various adaptation mechanisms to low temperatures by comparing the better adapted Saccharomyces species. The viability, vitality, fermentation capacity, and lipid composition of different Saccharomyces species (S. cerevisiae, S. bayanus, S. uvarum, and a hybrid S. cerevisiae/S. uvarum) with different fermentative origins (wine, beer, and baker’s strains together with a laboratory strain) were compared after culturing at low (13 °C) and optimal (25 °C) temperatures. In spite of specific responses of the different strains/species, the results showed that at low temperature, the medium-chain fatty acid and the triacylglyceride content increased, whereas the phosphatidic acid content and the phosphatidylcholine/phosphatidylethanolamine ratio decreased. Only the laboratory strain was not able to ferment the sugars, and after growing at both temperatures, its lipid composition was very different from that of the other strains. The hybrid strain showed the highest sugar consumption at 13 °C and the best vitality whatever the preculture temperature used. The rest of the species needed a preadaptation at low temperature involving a change in their lipid composition to improve their fermentation rate at 13 °C.     

Inhibition of Yersinia enterocolitica by Lactobacillus sake strains of meat origin

The growth of Yersinia enterocolitica at 4, 8, 15 and 24°C, in mixed cultures with Lactobacillus sake strains previously isolated from Spanish dry fermented sausages was investigated. Growth of Y. enterocolitica was affected by L. sake strains at all temperatures studied. The inhibition was higher as the incubation temperature increased. L. sake 148, a bacteriocinogenic strain, was less inhibitory to Y. enterocolitica growth than L. sake 23, a stronger lactic acid producer strain. The low pH and the lactic acid produced by the lactobacilli seem to be major factors contributing to the inhibition of Y. enterocolitica strains.     

Variability in biofilm production by Listeria monocytogenes correlated to strain origin and growth conditions

This study aimed to identify factors that influence the development of biofilm by Listeria monocytogenes strains and to determine the extent to which biofilm production protects against quaternary ammonium compound (QAC) disinfectant challenge. A total of 95 L. monocytogenes strains were studied and biofilm production was assessed as a function of incubation temperature, media pH, strain origin, serotype, and environmental persistence status. Attachment and biofilm development (inferred by the level of attached biomass) were measured in vitro using a colourimetric 96-well microtitre plate method in nutritive media (Brain-Heart Infusion). Increased biofilm production correlated with increasing temperature and the most acidic, or most alkaline, growth conditions tested. Clinical and environmental (food factory) strains were observed to increase biofilm production at higher and lower incubation temperatures respectively, independent of their rate of planktonic growth. Serotype 1/2a strains produced significantly more biofilm. Biofilm maturity, rather than strain, was correlated with resistance to QAC. Carbohydrate containing exopolymeric material could not be detected in the biofilm of representative strains, and no correlation between strains recovered as persistent food factory contaminants and biofilm production was identified. Although limited to in vitro inference based on the assay system used, our results suggest that environmental conditions determine the level of biofilm production by L. monocytogenes strains, independent of the rate of planktonic growth, and that this may manifest from selection pressures to which a given strain grows optimally.     

烟草疫霉菌拮抗细菌的筛选鉴定及发酵条件优化

为了筛选对烟草疫霉菌具有拮抗作用的生防细菌菌株,本研究从烟草黑胫病病圃地采集健康烟株的根际土样,采用对峙培养法筛选得到8株对烟草黑胫病具有拮抗作用的细菌菌株,其中编号为LB-9的菌株对烟草疫霉菌的抑制作用最强且效果稳定,抑菌率为60.6%,且抑菌谱广,对其进行16S rRNA基因序列分析后将该菌株鉴定为多粘芽孢杆菌。采用单因素优化试验初步探究了多粘芽孢杆菌LB-9的最适发酵条件,明确了LB-9的最适发酵培养基、最适发酵温度、最适发酵时间和最适发酵初始pH值,对其生防制剂的开发应用具有重要意义。     

Growths kinetics comparison of clinical and seafood Listeria monocytogenes isolates in acid and osmotic environment

Comparison of pathogenic bacterial strains of clinical origin with strains of the same species isolated from the environment may be a valuable tool for microbial risk assessment, especially for foodborne pathogens. Thus, a number of Listeria monocytogenes strains responsible for human cases of listeriosis, in relation to the consumption of contaminated seafood, have been compared with "natural" L. monocytogenes strains isolated from similar seafood products. Complete factorial designs were used to assess quantitatively the growth abilities of four clinical and four seafood isolates of L. monocytogenes placed in various environmental conditions. The cells were submitted to acid and osmotic stress as they were in stationary phase (constant condition) or in exponential phase (dynamic condition). The effects and interactions of pH (5-7) and NaCl concentration (0.5-8% v/v) were studied at two growth temperatures (10 and 20 degrees C). Growth parameters (lag and generation times calculated with Gompertz equation) were used to compare the behavior of the strains with respect to the conditions of culture. The results indicated an overall weak effect of acid stress alone, whereas osmotic stress clearly affected bacterial growth and a synergic effect between these two factors was observed. Clinical strains displayed better adaptation than seafood strains in stationary phase, however, this difference was not verified in exponential phase. Low temperature (10 degrees C) usually confirmed the observations at 20 degrees C, and the differences between clinical and food strains were more pronounced. Finally, a classification of the eight strains, based on the collected data, showed three groups: (i) seafood strains, (ii) three clinical strains and (iii) the last clinical strain, alone due to its high resistance to adverse conditions.     

Comparative life history studies of four Callosobruchus species infesting cowpeas with special reference to Callosobruchus rhodesianus (Pic) (Coleoptera: Bruchidae)

A comparative study of the ecology of Callosobruchus analis (F.), C. chinensis (L.), C. rhodesianus (Pic) and two strains of C. maculatus (F.) was made at three temperatures (25, 30 and 35°C) and three relative humidities (60, 70 and 80% r.h.). The performances of C. rhodesianus and the strain of C. maculatus from Brazil were also compared at 20°C. The intrinsic rates of increase of each species at each temperature were estimated. The optimum temperature for multiplication of all species was around 30°C. Total oviposition was highest at 30°C for all species, but survival from egg to adult was highest at 25°C for C. rhodesianus and the Malawi strain of C. maculatus compared with 30°C for the others. Development of both strains of C. maculatus was fastest at 35°C, but all other species developed most rapidly at 30°C. Relative humidity in the range 60–80% had no appreciable effect on any of the biological characteristics studied. The performance of C. rhodesianus was better at 20°C than that of the Brazilian strain of C. maculatus, and the results are discussed in relation to the distribution of the two species in Africa.